In recent years there has come into use a slim or trim style of venetian blinds in which the slats and headrail are generally about 1" in width or even slightly less. In keeping with the slim styling the lifting cords, which raise and lower the blind, are of relatively lightweight and small in diameter, being approximately 1/16th of an inch in diameter in many cases. The locking mechanisms for locking the cords in any adjusted position of the blind from fully raised to fully lowered generally comprise a housing mounted in the headrail. Within the housing there is rotatably mounted a guide roller. A second roller or locking pin, generally serrated about its circumference, is also mounted in the locking mechanism housing in a pair of spaced tracks or slots. The locking pin being loosely mounted in the slots of the housing may rotate with respect to the housing and may also move along the slots or tracks between two limit positions. In the first limit position the locking pin is closely adjacent to the guide roller and serves to grip the lifting cord between the guide roller and the serrated surface of the locking pin. When it is desired to adjust the blind the cord is pulled in one direction (downwardly) whereupon the locking pin drops to the second limit position in which it is spaced away from the guiding roller sufficiently to permit the lifting and lowering cord to move in either direction about the guiding roller to raise or lower the blind without interference from the locking pin. Upon release of the flexible cord the weight of the blind then attempts to pull the flexible cord upwardly and to drop to the fully lowered position of the blind; however, the flexible lifting and lowering cord, when pulled by the weight of the blind, engages the serrated surface of the locking pin and moves the short distance necessary to cause the locking pin to grip the cord between itself and the guide roller thus preventing a further downward movement of the blind. Thus, the weight of the blind, in attempting to drop down, can do so only to a very limited extent (usually only a fraction of an inch) before the cord is pinched and locked.
It will be appreciated that with such thin, lightweight cord, the tolerances with respect to the movement of the locking pin to and from the two limit positions and, importantly, the downward angle at which the flexible cord hangs from the guide roller, become critical and are very small. The weight of the very lightweight cord alone is what causes the engagement of the cord with the locking pin, causing it to move from the unlocked to the locked position. If for any reason the locking cord does not adequately engage against the locking pin when it is in its unlocked position, it will simply pass thereby under the weight of the blind until the blind is fully lowered. This situation is aggravated by the fact that such cords are conventionally of synthetic materials which tend to be glossy or slippery. Accordingly, it is very important that the guiding roller be so positioned, with respect to the locking pin, when the locking pin is in the unlocked position, as to guarantee engagement sufficiently between the cord and the locking pin.
For some time the guiding roller has been made of lignum vitae. This material has proven to be capable of accurate machining and to be relatively immune to changes in humidity. As such, the guiding roller has heretofore been readily capable of insuring the proper engagement between the lifting cord and the locking pin. However, in recent years lignum vitae has become increasingly impossible to obtain, and alternate materials have been substituted. Plastics, impregnated woods and the like, have been tried but have been found to be either too expensive or too variable dimensionally for any number of reasons including response to humidity and temperature to insure proper engagement of the cord with the locking pin. The problem is aggravated by the fact that attempts to cull out unsatisfactory rollers at the plant are not one hundred percent successful, and as a result customer complaints, once the device is in use, have tended to increase with attendant increases in costs for retrofitting with proper rollers.